weird yellow and red grains on my renders
These are often referred to as &quot;fireflies,&quot; and they're a common artifact in Cycles renders. Because Cycles is a path tracing engine which uses pseudo-randomness, some samples will find light sources and illuminate pixels that would otherwise be fairly dim.
You can either increase your number of samples (although it may take a very large number and long render time to eliminate them entirely), or you can take a look  at Blender's relatively new built-in denoising options.